covering of marine fouling. This is considered to be the reason some 

 investigators have found that, contrary to expectations, corrosion in 

 warmer waters has proceeded at practically the same rate as in cooler 

 waters. 



3. Oxygen Concentration 



Oxygen can affect the corrosion of steel in seawater in several 

 ways. It may cause variations in the electrical potential of metal areas 

 in a solution when its concentration varies along the metal surface; it 

 acts as a cathode depolarizer; it reacts with the ferrous atoms to form 

 oxides of the metal. Areas of low oxygen concentrations on a metal sur- 

 face are anodic to those of higher oxygen concentration (U. S. Army Corps 

 of Engineers, 19^2). 



Temperature and oxygen are interrelated in seawater corrosion. 

 Figure 1 shows variations in the corrosion rate of steel in air-saturated 

 and partially de-aerated sodium chloride solutions at various tempera- 

 tures. Seawater near the surface is nearly saturated with oxygen in 

 areas where considerable wave aqtion and spray exists (Fink, I960). 



k. pH Value 



There is little change in the corrosion rates of steel surfaces 

 between pH values of k and 9.5 at a given temperature. The surface is 

 in contact with a layer of hydrous ferrous oxide and corrosion can only 

 progress as fast as oxygen can diffuse the protective layer (U. S. Army 

 Corps of Engineers, 1962). As alkalinity increases from 9.5 5 the iron 

 tends to become passive as the permeability of the surface layer by 

 oxygen is decreased. As the pH value drops below k, the protective 

 corrosion product layer is dissolved and the acid reacts directly with 

 the metal. An example of the effect of pH value on corrosion rate is 

 shown in Figure 2. These curves were obtained by exposing mild steel 

 specimens to water having an oxygen concentration of 5 milliliters per 

 liter. Hydrochloric acid and sodium hydroxide were added as required 

 to produce the desired acidity and alkalinity for the investigation. 



5. Salinity 



Although the total salinity of seawater may vary in different 

 locations, the proportions of various salts relative to each other re- 

 main virtually the same (Fink, I96O). Of the various ions in seawater 

 resulting from the dissolved salts, the chloride ion is the most sig- 

 nificant. This is attributed to its being present in larger quantities 

 and to its ability to penetrate corrosion pupoduct films to continue its 

 activity in the corrosion process. Figure 3 shows the effect of sodium 

 chloride concentration on the corrosion rate of iron and the solubility 

 of oxygen as a function of salinity. Note that the concentration of 

 sodiiam chloride does not affect the oxygen solubility until a concentra- 

 tion of 5 to 10 grams per liter is reached. 



